A key subsystem in numerous optical systems for a variety of applications is an illumination system which comprises a light source, such as a laser or a lamp, and several optical components, such as mirrors and lenses, to collect, shape and relay the light from the source to the desired destination. For example, in a projector, light from an arc lamp is collected, made uniform, and made to illuminate an object, such as film or a programmable spatial light modulator, which is then imaged onto a display screen. As another example, in a laser lithography system, light from an excimer laser is collected, made uniform, shaped into a specific cross-section, is made to illuminate a photomask having a pattern, the mask being then imaged by a projection lens onto a substrate such as a semiconductor wafer or a display panel, coated with a layer of a photosensitive medium.
In all of these applications, the intensity of the light illuminating the object must be very uniform spatially. The object, as stated earlier, is, for example, a spatial light modulator (SLM) chip in a projector, or a photomask in a lithography system. Spatial uniformity of a light beam means that the cross-sectional profile of the intensity must be substantially flat. A second important requirement on the illumination system is that its efficiency must be as high as possible so that loss of light is minimized and the smallest possible lamp or laser light source may be incorporated in the optical system, or, alternatively, the highest possible energy may be obtained at the destination surface, such as the display screen or the semiconductor wafer.
Other highly desirable features in an illumination system include compact size and self-luminosity. The importance of a compact size of the illumination system is self-evident—it enables the whole optical system to be compact, and therefore, low-weight, more portable, etc. Self-luminosity of a light source means it is equivalent to an emission surface on which every point behaves effectively as an emission point from which light rays emanate in a specific numerical aperture. Such a characteristic is especially important when the illuminated object must be subsequently imaged with high resolution onto another surface. All of the above desirable features of illumination systems are important in the case of digital projections, lithography systems, and numerous other optical systems.
A widely used device for uniformizing the beam in an illumination system is a homogenizer in the shape of a light tunnel with a square, rectangular or hexagonal cross-section. Rays of an input light cone undergo multiple reflections between pairs of parallel mirror strips, causing random mixing of the rays and thereby uniformizing the beam. Such devices are employed in a variety of exposure systems and projectors.
In color projectors, the illumination beams of the three primary colors (red, green and blue) are produced by separating the broad-band (white) light of an arc lam or a halogen lamp by a segmented color wheel. Such a wheel, in a given position, allows only one of the colors to be transmitted, the others being blocked and lost. In a lithography system, the laser light incident on a mask is only partially utilized—only that portion of the light which falls on the transmissive regions of the mask is imaged by the projection lens and reaches the substrate. The majority of the light that falls on the opaque portions of the mask is blocked and lost.
An effective technique to minimize the loss of light described in the preceding paragraph is an “energy-recycling” homogenizer. Such a device has an apertured mirror at its input face. The cone of light from the source (lamp or laser) enters the homogenizer through the aperture and is uniformized by multiple reflections as in the conventional light-tunnel homogenizer. However, light rays reflected back from the color wheel (in a color projector) or the photomask (in a lithography system) are made to enter the homogenizer in the backward direction. These rays, when reaching the internal mirror surface surrounding the aperture at the input plate, are re-reflected to travel in the forward direction again, thus being re-utilized.
In all such illumination systems using a light-tunnel type of homogenizer, the longer the homogenizer, the more the number of reflections within, the greater the mixing of the rays, and therefore, the greater the uniformity of the output beam. However, an undesirable feature of such a homogenizer is that the size of the illumination system becomes larger. This is especially a significant disadvantage in the design of electronic projectors, for which small size of the illumination module is a highly attractive attribute, so that they can be made more compact, lower-weight, and more portable. This invention discloses novel illumination systems with very compact, high-efficiency, energy-recycling beam homogenizing devices. It also discloses compact homogenizing modules as beam-combining devices for efficiently combining the outputs of two or more light sources. Dividing a single input beam into multiple beams is also described.